The present invention relates to a method of preparing fine grain WC-Co(Ni)-powders for use in the manufacture of a cemented carbide.
WC-Co-cemented carbides are made by powder metallurgical methods of milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering. The milling operation is an intensive wet milling in mills of different sizes and with the aid of milling bodies which are usually made of cemented carbide. The milling time is of the order of several hours up to days. Milling is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed that the intensive milling creates a reactivity of the mixture which further promotes the formation of a dense structure.
During the long milling time, the milling bodies are worn and contaminate the milled mixture which has to be compensated for. The milling bodies can also break during milling and remain in the structure of the sintered bodies. Furthermore, even after an extended milling, a random rather than an ideal homogeneous mixture may be obtained. In order to ensure an even distribution of the binder phase in the sintered structure, sintering has to be performed at a higher temperature than necessary.
An alternative way is to start from an intimate mixture of cobalt and tungsten, which mixture subsequently is carburized. U.S. Pat. No. 3,440,035 discloses such a method of preparing cemented carbide powder characterized in that an aqueous solution or suspension of ammoniumparatungstate (APT) and nitric or hydrochloric aqueous solution of, e.g., cobalt, are mixed. The mixture is then subjected to a neutralizing reaction with ammonium hydroxide at a temperature of 20.degree. to 80.degree. C. and the pH-value of the mother solution after the reaction is adjusted to between 4.5 and 8. The resultant fine composite precipitate containing tungsten and cobalt in the desired composition controlled by reaction conditions is filtered, dried by heating and then subjected to reduction and carburization to obtain a WC-Co composite powder in which the WC grain size generally is submicron.
The solution pH is identified in U.S. Pat. No. 3,440,035 as a critical parameter for the control of the precipitation yields and, therefore also, control of the powder composition. The examples, however, indicate large variations in the solution pH leading to variations in the reaction yield and in the product composition. Precise control of composition is essential for the application of the method. The reactions should therefore be stoichiometric (99-100% yield) and/or have reproducible yields. Variations, as in U.S. Pat. No. 3,440,035, make accurate control of the powder composition impossible.